1. Field of the Invention
The present invention relates to a crankcase for a multiple cylinder engine, and more particularly, to a structure for providing breathing holes having a sufficient hole diameter in journal walls of the crankcase.
2. Description of Related Art
In a multiple cylinder engine, due to reciprocal movement of the pistons there are large variations in the internal pressure of the crank chamber during high-speed rotation of the crankshaft. As a result, friction loss becomes large. In order to solve this problem, breathing holes are formed which open into the journal walls. These breathing holes for each journal wall are formed by machine processing in a direction parallel to a central axis of the crankshaft with respect to a cast cylinder block; this results in each breathing hole being arranged coaxially (see, for example, Japanese patent laid-open No. Hei. 10-77905).
Oilways and bolt fastening sections are also provided in the journal walls. These oilways and bolt fastening sections are integrally formed when the cylinder block is casted. Therefore, the breathing hole diameter must be made small enough to avoid the oilways and bolt fastening sections. However, this makes it difficult to provide breathing hole diameters large enough to reduce friction loss.